Preparation of organic lead compounds



United States Patent ()fiice 3,007,857 Patented Nov. 7, l96l 3,007,857 PREPARATION OF ORGANIC LEAD. COMPOUNDS David G. Braithwaite, Chicago, Ill., assignor to Nalco Chemical Company, a corporation of Delaware N Drawing. Filed July 31,1957, Ser. No. 675,260

' 7 Claims. (Cl..204-59) This invention relates to the preparation of organic lead compounds, and more particularly to a. new and improved process for making tetraethyl lead.

The two principal methods which are employed to produce tetraethyl lead are described by the following equationswhich are designated, respectively, A and B:

Equation A represents one of the early synthesis processes which is no longer being used commercially. Equation B represents a current commercial process.

Both of these processes have serious disadvantages not the least of which is the lay-product production of free lead which is extremely toxic. Great care must be taken in the operation of these processes to avoid serious injury to workers. Furthermore, the yields of tetraethyl lead are not all that might be desired.

One of the objects of the present invention is to provide a new and improved process for producing organic lead compounds and especially tetraethyl'lead.

Another object of the invention is to provide a process for producing organic lead compounds in relatively high yields. Other objects will appear hereinafter.

In accordance with the invention it has been found that organic lead compounds can be produced by electrolyzing a Grignard reagent in an organic solvent for the Grignard reagent using a lead anode and a cathode which may be either lead, platinum or another conducting material. In this process the free hydrocarbon radicals derived from the Grignard reagent combine with the lead of the anode to form the corresponding organic metallo compound. The process may be described, therefore, as a sacrificial anode process.

In the preferred practice of the invention tetraethyl lead is produced by electrolyzing ethyl magnesium bromide dissolved in diethylether between two lead electrodes, the one forming the anode and the other the cathode. It will be understood, of course, that the cathode may be formed of platinum or another conducting material of the type usually used as an electrode. In this process, magnesium is deposited at the cathode and the free ethyl radicals which are liberated at the anode combine with the lead of the anode forming tetraethyl lead which dissolves in the diethylether and can be separated therefrom by distillation.

For commercial operations it is preferable to insert spacers between the anode and the cathode in order to segregate the magnesium at the cathode from the organic lead compounds formed at the anode. If no spacers are used the electrodes should be far enough apart so that the magnesium formed at the cathode does not bridge over to the anode.

In one form of process contemplated for the practice of the invention circulation of the solvent medium at the cathode is used to remove the magnesium which is filtered and reutilized for the formation of additional quantities of Grignard reagent.

The invention has been operated over a wide range of current densities and at varying voltages. The spacing and size of the electrodes will determine the current density.

Where a relatively low boiling solvent is employed, such as diethylether, the temperature used during the process is normally the boiling temperature of the sol- 2; vent. However, the process can be carried out at varying temperatures and higher boiling solvents such as tetrahydrofuran have been successfully used.

The invention is not limited to any particular solvent except that the solvent must be relatively inert under the conditions of the process. The solvents used are normally organic liquids which have dielectric properties but have sufficient conductivity to permit passage of'the current between the anode and the cathode. When the process is operated using solvents such as diethylether and tetrahydrofuran, organic lead compounds, such as tetraethyl lead are soluble therein. However, the process can be carried out with solvents for the Grignard reagent in which the organic lead compound is insoluble, Other liquidalkyl' ethers, such as dimethylether, diisopropylether and homologues thereof, including the polyoxyalkylene diethers can be employed as solvents ('e.g'., diethers of polyoxyethylene glycol's and diethers of poly oxypropylene glycols). The solvents used are'preferably anhydrous or substantially anhydrous.

The invention will be further illustrated by the follow: ing examples.

Example I Ethyl magnesium bromide was dissolved, in diethyle ether to produce a .67 N solution. Leadelectrodes which were 2 inches in diameter were placed in said solution spaced 1% inches apart and a direct current 05 300volts was applied at a current of 100 milliarnperes. The app a u s q ipped with a re l x ondenser to turn to thesolution any solvent which evaporated as a result of the boiling of the reaction mixture. Tetraethyllead was formed and the anode was completely used upl in the reaction. The normality of the solution was. reduced to 3 N-q i Example II 1400 cc. of diethylether solution containing 3.16 gram mols of ethyl magnesium bromide was electrolyzed by passing a direct current of .5 ampere through the elec trodes employing a voltage of 100 volts. The electrolysis was continued until 1.04 mols of the ethyl magnesium bromide was electrolyzed. Theoretically this should have prepared 0.251 mol oftetraethyl lead which would mean that 81 grams of tetraethyl lead'should have been present in the 1400 cc.

800 cc. of the electrolyzed solution was hydrolyzed by adding it dropwise to 400600 cc. of water in a B-neck flask provided with a stirrer and a reflux condenser. The diethylether which was volatilized by the heat of the reaction was returned to the reaction mixture by the reflux condenser. Two layers were formed, the bottom layer containing a cnrdy precipitate of magnesium hydroxide. Enough dilute sulfuric acid solution (lO-15% by weight H was added to the water layer to dissolve the curd. The water layer was separated and the diethylether layer containing the tetraethyl lead was distilled. 33.9 grams of a liquid having a boiling point of -102 C. at 30 mm. pressure was obtained. The liquid contained lead and had a specific gravity of 1.62. It can be concluded, therefore, that the electrolysis produced tetraethyl lead and only traces of materials other than tetraethyl lead.

Since the procedure of Example II was carried out at a conductance of approximately 0.5 of an ampere employing 100 volts and the procedure of Example I was carried out at a higher voltage and a lower amperage, it can be concluded that the current density can be varied rather widely.

The pressures used can also be varied and may be subatmospheric, atmospheric, or superatmospheric. The pressures used are normally sufficient to maintain the liquid phase with the particular solvent and temperature conditions employed.

It will be recognized that while the examples illustrate the preparation of tetraethyl lead other organic lead compounds can be prepared by substituting other Grignard reagents for the ethyl magnesium bromide. For example, isopropyl magnesium bromide, butyl magnesium bromide, amyl magnesium bromide, the corresponding iodides and chlorides and higher alkyl homo logues can be employed in place of ethyl magnesium bromide. Similarly, phenyl magnesium bromide or mixtures of phenyl and ethyl magnesium bromide can be electrolyzed to produce organic lead compounds containing the phenyl radical or both phenyl and organic radicals or both phenyl and other alkyl radicals in case a higher alkyl magnesium bromide issubstituted for ethyl magnesium bromide. t

The invention makes it possible to prepare organic lead compounds and especially substances such as tetraethyl lead by a very simple procedure in which free lead is not a by-product of the reaction. The process of the invention also makes it possible to produce organic lead compounds including tetraethyl lead in relatively high yields.

The invention is hereby claimed as follows:

1. A process for preparing organic lead compounds which comprises electrolyzing, using a lead anode, a substantially anhydrous solution of a Grignard reagent in a substantially inert organic solvent for said Grignard reagent employing an electrolyzing current effective to cause said lead anode to dissolve in said solution of said Grignard reagent in said organic solvent, said current being a minimum of 100 milliamperes, and recovering from the resultant product an organic lead compound consisting. of hydrocarbon radicals linked directly to metallic lead.

2. A process for preparing tetraethyl lead which comprises electrolyzing, usinga lead anode, a substantially anhydrous solution of ethyl magnesium bromide in a substantially inert organic solvent for said ethyl magnesium bromide employing an electrolyzing current effective to cause said lead anode to dissolve in said solution of said Grignard reagent in said organic solvent, said current being a minimum of 100 milliamperes, and recovering tetraethyl lead from the resultant solution.

3. Aprocess for preparing tetraethyl lead which comprises electrolyzing, using a lead anode, a substantially anhydrous solution of ethyl magnesium bromide in diethylether employing an electrolyzing current efiective to cause said leadanode to dissolve in said solution of said Grignard reagent in said organic solvent, said current being a minimum of 100 milliamperes, and recovering tetraethyl lead from the resultant solution.

4. A process for preparing tetraethyl lead which cornpnises electrolyzing, using a lead anode, a substantially anhydrous solution of ethyl magnesium bromide in tetrahydrofuran employing an electrolyzing current elfective to cause said lead anode to dissolve in said solution of said Grignard reagent in said organic solvent, said current being a minimum of milliamperes, and recovering tetraethyl lead from the resultant solution.

5. A process for preparing organic lead compounds which comprises electrolyzing between electrodes, one of which is a lead anode, a substantially anhydrous solution of a Grignard reagent in a substantially inert organic solvent for said Grignard reagent employing an electrolyzing current effective to cause said lead anode to dissolve in said solution of said Grignard reagent in said organic solvent, said current being a minimum of 100 milliamperes, the said electrodes being separated from each other in such a manner that the magnesium formed at the cathode does not bridge over to the anode, and recovering from the resultant product an organic lead compound consisting of hydrocarbon radicals linked directly to metallic lead.

6. A process as claimed in claim 5 in which the Grignard reagent is ethyl magnesium bromide and tetraethyl lead is recovered from the resultant solution.

7. A process .for preparing organic lead compounds which comprises electrolyzing, using a lead anode, an anhydrous solution of a Grignard reagent in tetrahydrofuran employing an electrolyzing current eifectiveto cause said lead anode to dissolve in said solution of said Grignard reagent in tetrahydrofuran, said current being a minimum of 100 milliamperes, said Grignard reagent being an ethyl magnesium halide, and recovering tetraethyl lead from the resultant solution.

References Cited in the file of this patent UNITED STATES PATENTS 2,849,349 Ziegler et a1 Aug. 26, 1958 FOREIGN PATENTS 534,128 Belgium Nov. 26, 1955 OTHER REFERENCES Chemical Reviews, vol. 54 (October 1954), 

1. A PROCESS FOR PREPARING ORGAINIC LEAD COMPOUNDS WHICH COMPRISES ELECTROLYZING, USING A LEAD ANODE A SUBSTANTIALLY ANHYDROUS SOLUTION OF A GRIGNARD REAGENT IN A SUBSTANTIALLY INERT ORGANIC SOLVENT FOR SAID GRIGNARD REAGENT EMPLOYING AN ELECTROLYZING CURRENT EFFECTIVE TO CAUSE SAID LEAD ANODE TO DISSOLVE IN SAID SOLUTION OF SAID GRIGNARD REAGENT IN SAID ORGANIC SOLVENT, SAID CURRENT BEING A MINIMUM OF 100 MILLIAMPERES, AND RECOVERING FROM THE RESULTANT PRODUCT AN ORGANIC LEAD COMPOUND CONSISTING OF HYDROCARBON RADICALS LINKED DIRECTLY TO METALLIC LEAD. 